The present invention relates in general to telephony, and in particular to circuits for supplying dc current to telephone lines.
In one type of telephone line, such as a subscriber loop, dc current as well as ac voice signals are necessary for the transmission of voice intelligence. A telephone set connected to such loop requires dc current for the proper operation of the talking transmitter.
Owing to the fact that each subscriber loop requires a dc supply circuit, and large switching systems service up to 10,000 subscriber loops, such supply circuits should optimally be very efficient in transferring dc power to the line rather than dissipating power internally within the circuits. Moreover, the supply circuit elements contributing to the internal power consumption that does exist should be low cost and of physically small size so as to conserve circuit board or rack space.
It is well known in the art that the simplest line supply circuit consists of a pair of equal-valued resistors each placed in series with the respective loop tip and ring conductor. Such an arrangement requires physically large power resistors which, for short loop lengths, dissipate more power than is fed to the loop itself.
Filament lamp supply circuits represent an advantage over the resistor circuits insofar as the nonlinear temperature vs. resistance characteristics reduce internal power consumption when used in conjunction with short loops. However, these lamps are expensive, require significant space, and are not easily obtained.
Other solid state line supply circuits have been devised to feed dc current to telephone lines. Such circuits, one of which is disclosed in U.S. Pat. No. 3,649,769, exhibit linear current feed characteristics similar to the traditional resistor circuit and share the same disadvantage as noted before.
Many of the foregoing shortcomings are addressed in U.S. Pat. No. 4,056,691 which describes an efficient telephone subscriber line circuit for feeding a constant current to a subscriber set the magnitude of which is essentially independent of loop length. The basis of that feed circuit is a complicated and sophisticated switching regulator. While that arrangement achieves its objectives, it becomes economically impractical to utilize a number of such circuits in a system servicing more than several hundred subscriber loops.
As the sophistication of telephone line supply circuits increases, especially toward solid state units, impedance considerations must be observed when used in an environment which requires a balanced relationship between the tip and ring conductors. Solid state supply circuits generally exhibit a type of impedance which does not permit the supply to be inserted within the voice path of the telephone line. Therefore, as in noted U.S. Pat. No. 3,649,769 the supply circuit shunt feeds the telephone line and is accordingly designed to maintain a high impedance to the voice signals.
Accordingly, it is a primary object of the present invention to provide an economical and efficient battery supply circuit for feeding dc current to telephone lines as a function of line length, and for dissipating power in the circuit within low cost and moderate size components.
It is a related object of the present invention to provide a telephone line feed circuit wherein the internal power dissipation can be arbitrarily divided between a resistor and an transistor by the selection of the resistor value.
It is a further object of the present invention to provide a series subscriber loop regulator having a low ac impedance to thereby minimize longitudinal impedance imbalances between the telephone conductors.
The foregoing, as well as other objects and advantages of the present invention will become apparent from the detailed description of the invention which follows hereinafter, and when considered together with the appended drawings.
While this invention will be described in connection with certain embodiments presently considered to be preferred, there is no intent to limit it to these embodiments. On the contrary, the intent is to cover all alternatives, modifications and equivalents included within the spirit and scope of the invention as defined by the appended claims.